A solution of ferric subsulfate has been recognized as a medicament used for its astringent/styptic properties since at least as early as 1960 when it was listed in National Formulary XI. It has been listed there and in subsequent editions as ferric subsulfate solution or Monsel's Solution or basic ferric sulfate solution. It is there identified as a reddish-brown liquid, nearly odorless, with a sour, strongly astringent taste. It is acid to litmus, is affected by light, and has a specific gravity of about 1.548. As referenced in Remington's Practice of Pharmacy, 13th ed., ferric subsulfate solution is stated to be prepared by the oxidation of ferric sulfate nitric acid. It is an important styptic solution because it is less irritating than ferric sulfate because of the lesser amount of sulfuric acid present.
In practice, a ferric subsulfate solution is prepared by the oxidation of ferrous ferric sulfate with nitric and sulfuric acids. Listed below is an equation that is believed to approximate that used to form ferric subsulfate. EQU 12 FeSO.sub.4 +4 HNO.sub.3 +3 H.sub.2 SO.sub.4 .fwdarw.3 Fe.sub.4 (OH).sub.2 (SO.sub.4).sub.5 +4 NO+2H.sub.2 O
When maintained in aqueous solution, ferric subsulfate presents definite problems of usage. Ferric subsulfate is miscible with water. As a consequence, the reacted solution may also contain remnants of the nitric and sulfuric acids used to form the compound, and it will be apparent that any excess nitric and/or sulfuric acid in the solution will greatly enhance the skin irritating characteristics of the solution. Additionally, the solution sporadically crystallizes and precipitates at room temperature, which characteristic is enhanced at temperatures below room temperature. Even upon redissolving of the precipitated or crystallized subsulfate by heating, the official solution is murky rather than clear; so, it is difficult to determine whether the crystallized or precipitated ferric subsulfate has been completely dissolved or merely dispersed in the aqueous medium. Indeed, as marketed, e.g., by Spectrum Mfg. Co., of Gardena, Calif., there is a notation that the product may crystallize.
As a consequence, it is difficult without titrating the solution to determine whether a ferric subsulfate solution is of the proper strength. Such an assay is described in National Formulary XI, titration being carried out against sodium thiosulfate after the addition of HCl and KI. The official solution contains 20 to 22 grams of iron per 100 ml. Thus, it would be advantageous to utilize a form of ferric subsulfate which requires a lesser concentration of the active ingredient, thereby mitigating adverse side effects caused by the ferric subsulfate.
It is, therefore, a primary object of the present invention to provide a ferric subsulfate composition that has substantially the same astringent and styptic properties of Monsel's Solution, but is less irritating than Monsel's Solution to the skin, mucous membrane, and other target tissue.
It is another object of the present invention to provide a ferric subsulfate astringent in a form in which the amount of iron-containing subsulfate may be accurately maintained and applied to target tissues, and in which the concentration of iron is substantially less than the 20 to 22% iron of Monsel's Solution.
It is still another object to provide ferric subsulfate in a form in which it can be assured that there is a substantial absence of the nitric and sulfuric acids used to prepare the subsulfate.